Oil and gas producing formations occur as microporous strata and the production rate for flow of oil or gas into a well bore depends on available pressure differentials in the stratum and the stratum's permeability or porosity. After a bore hole has been drilled to or through an oil or gas producing stratum, it must, in order to become a reliably producing well, be subjected to a completion operation. This operation insures that the oil or gas, through the useful life of the well, is able to flow freely into the well bore and reach the surface.
Such operations vary substantially in basic type and detail, depending on a number of factors including the type of stratum being tapped, e.g., unconsolidated sand, sandstone or porous limestone.
One type of completion operation is that referred to as an open hole completion. In this procedure, an iron-cased and cemented bore hole is dug to the top of the stratum, after which penetration into the stratum is achieved by underreaming, that is, drilling an oversized hole below the lined bore hole into and through the producing stratum. This large diameter open hole is then stabilized by packing it with gravel around a slotted screen at the center of the open hole. The slotted screen is connected to the cased bore hole. Oil or gas flow is then permitted or induced from the stratum into the gravel pack, and then to and through the well bore.
A second type of completion operation involves drilling the bore hole through the stratum, after which it is cased and cemented. Access to the producing stratum from the well bore is then achieved by perforating the casing cement wall by means of shaped charges. The perforation holes may then be gravel packed if, for example, the formation is an unconsolidated sand.
In these and most other well completion operations, a hydrostatic balance is maintained in the well to prevent oil or gas flow from the formation until desired by using a fluid column usually comprised of a water-based brine composition of appropriate density in the well bore. This fluid also serves to clean contaminants from previous operations, e.g., drilling mud from the well bore, and to transfer gravel to the well bore. During these completion operations, the completion fluid becomes laden with a wide variety of suspended solid particulates comprised of drilling and formation debris, such as gravel, sand, ground up rock from the drilling operation and clay particulates, particularly bentonite, from the drilling mud. These particulates typically range in size from gravel, i.e., small rocks, down to submicronic particles of 0.1 micrometer in size or even smaller. Those of about 0.1 micrometer to about 30 micrometers in size pose a particular threat to the permeability of the producing formation, especially adjacent the well bore, such as at an open hole surface or at the perforations referred to above.
The reason for this is that, when the completion fluid penetrates into the formation as it does under conditions of well bore over-pressure, it carries with it those particles small enough to enter the formation pores and these particles are then deposited in these pores, plugging the formation. Specific operations, such as the perforation wash, in which completion fluid is jetted through the perforations into an unconsolidated sand formation to clean the formation, is an extreme example of where even a low level of small particulate contamination in a completion fluid can severly reduce the formation permeability in its most critical region, that is immediately adjacent the well bore.
Two strategies are commonly used to avoid such formation damage. One is to use only exceptionally clean (i.e, particle-free) fluids, where, in practice, an upper size limit for particles present in the completion fluid is, for example, 1 micrometer. Another is to have a well-controlled dispersion of particles that, in the initial stage of the completion operation, will jam and form an impermeable but removable cake on the stratum surface at the well bore, thereby preventing entry into the stratum of both fluid and suspended particles during the completion operation. This cake is then removed by use of an acid or the like when the completion operation is completed and the well is ready to be brought on stream.
Both these strategies involve the ability to completely remove contaminant particles of greater than, for example, 1 micrometer in size from a completion fluid prior to its injection into the well. This is difficult and made more so in those completion operations where the fluid is recirculated many times through the well, emerging each time with an added contaminant load.
In such an operation, it may be necessary to remove dirt loadings of up to 10 percent by weight from the completion fluid, much of this, however, being of such a size as to be removable by conventional on-line processes, such as screening or centrifuging. For particulates smaller than 30 micrometers, however, such devices are ineffective and will leave solid particulates of between 0.1 and 30 micrometers in the completion fluid at levels that may reach 10,000 parts per million of solids on a weight basis (hereinafter "ppm"). Within this particle size range, i.e., from 0.1 to 30 micrometers, are precisely those particles which are most capable of invading the pores of a typical oil or gas bearing stratum and significantly reducing its permeability.
Reduction of stratum permeability leads to significant losses in well productivity. Indeed, many wells, upon completion, show little production. The wells must then be subjected to recompletion, in hopes of a better job, or stimulation by acidizing or fracturing at the cost of increased expense and additional down time. Similarly, productivity profiles of producing wells decline over time. When productivity reaches a certain minimal value, remedial rework (acidizing, fracturing or recompletion) must be attempted. Wells completed with a high quality (clean) fluid not only show a higher initial productivity, they also have a slower decline in production profiles, hence lengthening the time intervals between remedial workover.
Substantially complete removal of solid particulates in the particle size range of from about 1 to about 30 micrometers has not previously been economically feasible using available cartridge filtration technology. Cartridge filtration practice as applied prior to the subject invention, primarily using depthtype filters, typically cylinders with a hollow core with walls of about three-quarters of an inch in thickness and made of wound or randomly laid fibers, such as polypropylene and fiberglass, only resulted in a reduction of the contaminant loading of particles. To achieve even moderate levels of clarity, e.g., 80 ppm in the effluent from an influent loading of 2,000 ppm, would require a number of passes through the filter system using previously available depth filter technology. Substantially complete removal of particles above 0.1 micrometer was not a realizable goal with prior art filtration technology, thus not permitting the desired control of solids in a recycled completion fluid, as required by the two strategies discussed above.
Because of the high loading in the influent treatment fluid, conventional cartridge filters using pleated filter elements operating at conventional flow rates, e.g., from 1 to 4 gallons per minute per square foot of filter surface, quickly became clogged and developed unacceptable pressure drops, rendering them economically unsatisfactory. The cost of such filters, compared with depth filters, and their short life when operated at conventional flow rates, leading to excessive downtime for changeout, has substantially precluded their use in the oil industry for filtering recirculating oil and gas well treatment fluids.
This invention, then, is directed to a filtering process for obtaining a clear effluent filtrate from a turbid oil or gas well treatment fluid contaminated with solid particulates comprised of drilling and formation debris having particle sizes in the range of from about 0.1 to about 30 micrometers and where the effluent filtrate is substantially free of solid particulates having particle sizes in the range of from about 0.1 to about 30 micrometers.